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Peculiar Near-Earth Objects Mem. S.A.It. Suppl. Vol. 6, 86 Memorie della c SAIt 2005 Supplementi Peculiar Near-Earth Objects S. Marchi1 M. Lazzarin1 and S. Magrin1 Dipartimento di Astronomia, Universita` di Padova, Italy e-mail: [email protected], [email protected], [email protected] Abstract. In this paper we present visible and near-infrared spectroscopy of 7 peculiar NEOs. These objects have been observed within the SINEO (Spectral Investigation of Near- Earth Objects) survey, which have produced more then 100 spectra of NEOs. Among the objects presented here, five of them result to be spectrally peculiar, belonging to the rare V and R classes. The other two have peculiar orbits being comet-like. Their spectroscopy gives a B- and a X-type which is compatible with the scenario of being objects which originated in the outer Solar System. Key words. minor planet – asteroids – near-Earth objects 1. Introduction bodies and two having comet-like orbits and a primitive composition. Much progress in understanding the origins of near–Earth objects (NEOs) has been accom- plished in the last three decades. NEOs have 2. Igneous bodies in general chaotic, short–living orbits (typical life–times of the order of a few million years), 2.1. V-type suggesting that they have to be continuously In figure 1 the visible spectra of four V-types resupplied from an external reservoir. Two ma- observed NEOs are shown. For each body, the jor sources have been identified (for a review spectrum of the asteroid 4 Vesta has been over- see Morbidelli et al. (2002)). The principal plotted for a direct comparison. one is the Main Belt, where gravitational per- They exhibit an excellent spectral match turbations, mainly by Jupiter and Saturn, cre- with Vesta. We also report the range of varia- ate dynamically resonant regions which pro- tion concerning the previously defined V–type vide escape routes (in this case we refer to asteroids on the basis of 39 V–type objects near–Earth asteroids, NEAs). The second (by found within SMASSII (Bus 1999). The pre- far less important) source is represented by ex- sented NEAs are significantly more similar to tinct comets: a certain number of NEOs may 4 Vesta than the previously known V–type MB represent the final evolutionary state of comets, asteroids. The same is true within the sample of i.e. a devolatilized nucleus. V–type NEAs (a total of 15 bodies, see Binzel In this paper we present the spectroscopy of et al. (2002)). The main differences between some peculiar NEOs: five peculiar igneous 4 Vesta and the other V–types are the slopes in the range 0.4-0.72 µm (from moderately to Send offprint requests to: S. Marchi much redder than Vesta) and the depth of the 1 Marchi: Peculiar NEOs 87 µm band (deeper than Vesta’s: see Florczak et al. (2002); Burbine et al. (2001); Binzel & Xu 2.5 (1993)). We are aware that neither differences 2003 GJ21 in slope do necessarily indicate differences in 2 2003 FU3 composition, as shown by Kelley et al. (2003), nor the similar spectral behaviour between two 2003 FT3 objects necessarily establishes a genetic link. 1.5 However, the similarity between 4 Vesta and Reflectivity 2003 EG the NEOs, whose observations are presented in 1 this paper, stands as a striking feature. For this V-type max V-type min reason, in the following we shall compare our (4) Vesta 0.5 data not only to V–types in general, but also of- 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength (µm) ten to Vesta. For two NEAs in our sample, namely 2003 FT3 and 2003 EG, we performed also NIR ob- servations; unfortunately these spectra are too 2003 FT3 noisy for a quantitative analysis of bands posi- 1.4 (4) Vesta (2579) Spartacus tion and depth, which would be necessary for a detailed compositional analysis. 1.2 The composite visible-NIR spectrum of 1 2003 EG (Fig. 1, lower panel) resembles that Reflectivity of 4 Vesta throughout all the observed range, 0.8 and in particular the shape of the 1 µm band is similar. However, the depth of the 1 µm band 0.6 is not the same, being deeper on Vesta than on 0.5 1 1.5 2 2.5 2003 EG, perhaps indicating a different con- µ Wavelength ( m) tent of pyroxene and/or olivine. Unfortunately, the 2 µm band is out of the measured range, 1.8 thus preventing any detailed compositional analysis. Anyway, the high reflectivity level of 1.6 2003 EG 2003 EG at 1.6 µm suggests a lower content of (4) Vesta 1.4 pyroxene with respect to Vesta. The overall NIR spectrum of 2003 FT3 (Fig. 1.2 1, middle panel) is consistent with the V–type, Reflectivity 1 but the right–hand limb of the 1µm band is very peculiar and much different from other 0.8 V-types (and from Vesta as well). Moreover the 2 µm band is less deep than that of Vesta. 0.6 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Wavelength (µm) Similar features, for the first time detected among NEOs, have been observed at least Fig. 1. Upper panel: Comparison between the ob- in another MB V–type object, namely 2579 served NEOs (namely, 2003 EG, 2003 FT3, 2003 FU3, and 2003 GJ21) and 4 Vesta in the visible re- Spartacus (Burbine et al. 2001). We stress that gion. The spectra are shifted vertically for clarity. the spectra of Vesta, Spartacus and 2003 FT3 For 2003 EG the maximum and minimum spread- are rather different one from each other. ing among V-types (Bus 1999) is also shown. However, the broad 1 µm bands of Spartacus Middle panel: Comparison between 2003 FT3 and and 2003 FT3 may suggest a compositional 4 Vesta. The Main Belt asteroid 2579 Spartacus is similarity (note also that no similar features also overplotted (Burbine et al. 1997). Lower panel: have been detected among HED meteorites). Comparison between 2003 EG and 4 Vesta. The The peculiar spectral features can be seen smoothed specrtum of 2003 EG is also shown, for (here as for 2579 Spartacus, see Burbine a easier comparison. 88 Marchi: Peculiar NEOs havior of igneous, V-like, objects. However its visible spectrum also presents many common aspects with another group of igneous bodies, 1,4 Vesta MS-CMP-042-B (Laser irrad. oliv.) 0.6 Vesta + 0.4 Oliv. those belonging to the R-class. Tholen et al. 2003 FT3 1,2 (1989) introduced the distinction between V- and R-class on the basis of the spectra of two 1,0 peculiar MBAs, 4 Vesta and 349 Dembowska, Reflectivity respectively (for further details, see Gaffey et 0,8 al. (1993)). Recently Bus (1999), still kept this distinction, although he provided new 0,6 spectra in both classes increasing the spreading between each class. In particular he found 0,5 1,0 1,5 2,0 2,5 Wavelength (µm) 3 more objects similar to 349 Dembowska, and hence classified as R-types. However, a detailed inspection of all the visible spectra Fig. 2. Linear combination of Vesta’s spectrum with laser irradiated olivine. The percentages which give of V- and R-types show that this division is the best fit are 60% Vesta and 40% olivine (see text somewhat weaker than that introduced by for further details). Tholen. According to this classification, 2001 XR31 would be at the border between V- and R-class (see Fig. 3, upper panel). However, we et al. (2001)) as diagnostic of an excess underline also that the spectrum of 2001 XR31 of olivine and/or plagioclase, compared to surprisingly overplots exactly the R-types in typical V–types. Indeed, olivine has a broad the range 0.4-0.85 µm. Moreover, we point profile at the right side of the 1 µm band and out another similarity between the R-types has no 2 µm band (e.g. see Hiroi & Sasaki and our NEO, namely on the basis of their (2001)) while plagioclase has a wide band that behavior within the range 0.86-0.95 µm. In extends from 1 to 1.5 µm, and has no 2 µm fact, the reflectivity of all the R-type objects band. In order to investigate such hypotesis, in this interval is noticeably constant (see Fig. we computed a linear combination (e.g. see 3, upper panel) while for the V-types it is Hiroi et al. (1993)) of Vesta spectrum with generally slightly decreasing. In other words, the spectrum of olivine (taken from RELAB, the bottom of the 1µm band is almost flat. http://www.planetary.brown.edu/relab/). The Now, it is difficult to understand the origin of result is shown in fig. 2. Although the linear this behavior, but it is believed to be significant combination could represent a oversimpli- for compositional differences. We point out fication of the actual 2003 FT3 surface (it that it could be due to mixture of compounds holds only if the diverse species have separate which have the peak of the 1 µm band at locations on the surface), an excellent match slightly different wavelengths, for example by has been achieved. We recall that a spot of a mixture of olivine and pyroxene or a mixture olivine has been identified (Gaffey (1997); of pyroxenes with different contents of Fe (e.g. Thomas et al. (1997); Cochran & Vilas see Gaffey (1997)). To better show the dif- (1998)) on the surface of Vesta: 2003 FT3 ferences between V-types and R-types, in Fig.
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